Engine Management Systems are increasing in complexity to provide better performance and meet tighter vehicle emissions regulations. It is current practice for electronic modules used in an EMS to undergo a conventional process (in-circuit) test before conformal coating. After coating and assembly, functional test is performed at ambient and extreme temperature levels to verify that the module will maintain its performance over a full operating temperature range (-40.degree.C. to +125.degree. C.). When functionally testing EMS modules, additional access is provided by a diagnostic serial interface, which is designed for use during periodic servicing. This serial interface, usually based on the ISO9141 protocol in Europe or J1850 in the USA, can also be used to access diagnostic registers in the module during production test. Additional Built-In Self Test (BIST) software can be downloaded via this serial interface. When testing analogue inputs, e.g. air temperature, the usual technique is to provide a signal stimulus on the appropriate pin, send a command to the Unit Under Test (UUT) and read back an internal register which records the value generated by the ADC for the input under test. When testing outputs, e.g. stepper motor output, a command is sent to the UUT to turn on the relevant transistors, the output is connected to a suitable load and measurement instrument and then it is "driven" using an electronic load and power supply, or a four-quadrant V/I source. Gain values can be recalculated from external measurements and written to internal registers to correct for production tolerances.
However, even when tested in this manner, problems can occur when the EMS is completed and installed in an automobile. For example, EMSs, which have passed functional test, fail when installed in automobiles because of a change in a memory chip. One attempt to deal with this has been to fit EMSs inside an automobile at the end of the production line. The engine is then started and idled for a few minutes to verify that the EMS modules are functioning correctly. Failure can also occur after installation because the wrong operating software has been loaded, again after End of Line (EOL) functional testing has been completed. Another failure can occur when a diesel management system enters a "run away" condition in use, causing the vehicle speed to increase as fuel is pumped unchecked into the engine.
It is an object of the present invention to provide an "open loop" testing system which would allow such problems to be detected before the EMS module is installed in the automobile.